Android recovery 流程分析

Recovery簡介

Android利用Recovery模式，進行恢復出廠設置，OTA升級，patch升級及firmware升級。

升級一般通過運行升級包中的META-INF/com/google/android/update-script腳本來執行自定義升級，腳本中是一組recovery系統能識別的UI控制，文件系統操作命令，例如write_raw_image（寫FLASH分區），copy_dir（複製目錄）。該包一般被下載至SDCARD和CACHE分區下。如果對該包內容感興趣，可以從http://forum.xda-developers.com/showthread.PHP?t=442480下載JF升級包來看看。

升級中還涉及到包的數字簽名，簽名方式和普通JAR文件簽名差不錯。公鑰會被硬編譯入recovery，編譯時生成在：out/target/product/XX/obj/PACKAGING/ota_keys_inc_intermediates/keys.inc

G1中的三種啓動模式

MAGIC KEY:

camera + power：bootloader模式，ADP裏則可以使用fastboot模式
home + power：recovery模式
正常啓動

Bootloader正常啓動，又有三種方式，按照BCB（Bootloader Control Block, 下節介紹）中的command分類：

command == 'boot-recovery' → 啓動recovery.img。recovery模式
command == 'update-radio/hboot' → 更新firmware（bootloader)
其他 → 啓動boot.img

Recovery涉及到的其他系統及文件

CACHE分區文件

Recovery 工具通過NAND cache分區上的三個文件和主系統打交道。主系統（包括恢復出廠設置和OTA升級）可以寫入recovery所需的命令，讀出recovery過程中的LOG和intent。

/cache/recovery/command： recovery命令，由主系統寫入。所有命令如下：
- --send_intent=anystring - write the text out to recovery.intent
- --update_package=root:path - verify install an OTA package file
- --wipe_data - erase user data (and cache), then reboot
- --wipe_cache - wipe cache (but not user data), then reboot
/cache/recovery/log：recovery過程日誌，由主系統讀出
/cache/recovery/intent：recovery輸出的intent

MISC分區內容

Bootloader Control Block (BCB) 存放recovery bootloader message。結構如下：

struct bootloader_message {

char command[32];

char recovery[1024];

};

command可以有以下兩個值

“boot-recovery”：標示recovery正在進行，或指示bootloader應該進入recovery mode

“update-hboot/radio”：指示bootloader更新firmware
recovery內容

“recovery\n

<recovery command>\n

<recovery command>”

其中recovery command爲CACHE:/recovery/command命令

兩種Recovery Case

FACTORY RESET（恢復出廠設置）

用戶選擇“恢復出廠設置”
設置系統將"--wipe_data"命令寫入/cache/recovery/command
系統重啓，並進入recover模式（/sbin/recovery）
get_args() 將 "boot-recovery"和"--wipe_data"寫入BCB
erase_root() 格式化（擦除）DATA分區
erase_root() 格式化（擦除）CACHE分區
finish_recovery() 擦除BCB
重啓系統

OTA INSTALL（OTA升級）

升級系統下載 OTA包到/cache/some-filename.zip
升級系統寫入recovery命令"--update_package=CACHE:some-filename.zip"
重啓，並進入recovery模式
get_args() 將"boot-recovery" 和 "--update_package=..." 寫入BCB
install_package() 作升級
finish_recovery() 擦除 BCB
** 如果安裝包失敗 ** prompt_and_wait() 等待用戶操作，選擇ALT+S或ALT+W 升級或恢復出廠設置
main() 調用 maybe_install_firmware_update()
1. 如果包裏有hboot/radio的firmware則繼續，否則返回
2. 將 "boot-recovery" 和 "--wipe_cache" 寫入BCB
3. 將 firmware image寫入cache分區
4. 將 "update-radio/hboot" 和 "--wipe_cache" 寫入BCB
5. 重啓系統
6. bootloader自身更新firmware
7. bootloader 將 "boot-recovery" 寫入BCB
8. erase_root() 擦除CACHE分區
9. 清除 BCB
main() 調用 reboot() 重啓系統

Recovery模式流程

/init → init.rc → /sbin/recovery →

main():recovery.c

ui_init():ui.c ［UI initialize］
- gr_init():minui/graphics.c ［set tty0 to graphic mode, open fb0]
- ev_init():minui/events.c [open /dev/input/event*]
- res_create_surface:minui/resource.c [create surfaces for all bitmaps used later, include icons, bmps]
- create 2 threads: progress/input_thread [create progress show and input event handler thread]
get_args():recovery.c
- get_bootloader_message():bootloader.c [read mtdblock0(misc partition) 2nd page for commandline]
- check if nand misc partition has boot message. If yes, fill argc/argv.
- If no, get arguments from /cache/recovery/command, and fill argc/argv.
- set_bootloader_message():bootloader.c [set bootloader message back to mtdblock0]
Parser argv[] filled above
register_update_commands():commands.c [ register all commands with name and hook function ]
- registerCommand():commands.c
  - Register command with name, hook, type, cookie.
  - Commands, e.g: assert, delete, copy_dir, symlink, write_raw_image.
- registerFunction():commands.c
  - Register function with name, hook, cookie.
  - Function, e.g: get_mark, matches, getprop, file_contains
install_package():
- translate_root_path():roots.c [ "SYSTEM:lib" and turns it into a string like "/system/lib", translate the updater.zip path ]
- mzOpenZipArchive():zip.c [ open updater.zip file (uncompass) ]
- handle_update_package():install.c
  - verify_jar_signature():verifier.c [ verify signature with keys.inc key; verify manifest and zip package archive ]
    - verifySignature() [ verify the signature file: CERT.sf/rsa. ]
      - digestEntry():verifier.c [ get SHA-1 digest of CERT.sf file ]
      - RSA_verify(public key:keys.inc, signature:CERT.rsa, CERT.sf's digest):libc/rsa.c [ Verify a 2048 bit RSA PKCS1.5 signature against an expected SHA-1 hash. Use public key to decrypt the CERT.rsa to get original SHA digest, then compare to digest of CERT.sf ]
    - verifyManifest() [ Get manifest SHA1-Digest from CERT.sf. Then do digest to MANIFEST.MF. Compare them ]
    - verifyArchive() [ verify all the files in update.zip with digest listed in MANIFEST.MF ]
  - find_update_script():install.c [ find META-INF/com/google/android/update-script updater script ]
  - handle_update_script():install.c [ read cmds from script file, and do parser, exec ]
    - parseAmendScript():amend.c [ call yyparse() to parse to command ]
    - exeCommandList():install.c
      - exeCommand():execute.c [ call command hook function ]
erase DATA/CACHE partition
prompt_and_wait():recovery.c [ wait for user input: 1) reboot 2) update.zip 3) wipe data ]
- ui_key_xxx get ALT+x keys
- 1) do nothing
- 2) install_package('SDCARD:update.zip')
- 3) erase_root() → format_root_device() DATA/CACHE
may_install_firmware_update():firmware.c [ remember_firmware_update() is called by write_hboot/radio_image command, it stores the bootloader image to CACHE partition, and write update-hboot/radio command to MISC partition for bootloader message to let bootloader update itself after reboot ]
- set_bootloader_message()
- write_update_for_bootloader():bootloader.c [ write firmware image into CACHE partition with update_header, busyimage and failimage ]
finish_recovery():recovery.c [ clear the recovery command and prepare to boot a (hopefully working) system, copy our log file to cache as well (for the system to read), and record any intent we were asked to communicate back to the system. ]
reboot()